This is something I'm certain we've all done before, but I was amazed that there were so many ways you could go about this! The process is pretty straight forward, so I won't go into too much detail. Paint the textured side of the leaves with tempura or acrylic paint, then press the leaf to the paper. So simple, yet it provided nearly an hour of creativity and fun for my 4 year old.
Some variation ideas include:
White or silver acrylic paint on paper or fabric, then dye or watercolor in a dark color for contrast
Use flowers, fruit, vegetables, pine needles, feathers, etc...
Use a clothes pin and transform flower petals, grass, feathers and leaves into a paint brush!